Avoiding hot cracks during laser welding of a workpiece stack-up assembly of aluminum alloy workpieces

Abstract

A method of laser welding a workpiece stack-up that includes two or more overlapping aluminum alloy workpieces is disclosed. The method involves controlling the power level of the laser beam during at least one of an initial stage or a final stage of advancing the laser beam along a weld path so as to limit a line energy of the laser beam during such stage or stages to being no greater than 10% above a line energy of the laser beam during an intermediate stage of laser beam advancement that is performed between the initial and final stages. By limiting the line energy during the initial and/or final stages of laser beam advancement along the weld path, excessive fusion of the workpiece stack-up assembly can be avoided in those locations to help protect against hot-cracking in the resultant laser weld joint.

The invention claimed is: 
     
       1. A method of laser welding a workpiece stack-up assembly that includes at least two aluminum alloy workpieces, the method comprising:
 providing a workpiece stack-up assembly that includes at least a first aluminum alloy workpiece and a second aluminum alloy workpiece that overlap one another to establish a faying interface between the workpieces, the first aluminum alloy workpiece providing an accessible top surface of the workpiece stack-up assembly; 
 transmitting a laser beam at the accessible top surface of the workpiece stack-up assembly to create a keyhole within the workpiece stack-up assembly and a molten aluminum alloy weld pool that surrounds the keyhole, the keyhole and the molten aluminum alloy weld pool traversing at least the faying interface established between the first and second aluminum alloy workpieces, the laser beam having a power level; 
 advancing the laser beam relative to the accessible top surface of the workpiece stack-up assembly to convey an impingement point of the laser beam along a weld path that includes a beginning portion and an ending portion so as to translate the keyhole and the surrounding molten aluminum alloy weld pool along a corresponding route within the workpiece stack-up assembly, the laser beam being advanced along the weld path at a travel speed and, further, the laser beam having a line energy, which is defined by the equation E L =P L /S L , wherein E L  is the line energy of the laser beam, P L  is the power level of the laser beam, and S L  is the travel speed of the laser beam; and 
 controlling the power level of the laser beam during at least one of an initial stage or a final stage of advancing the laser beam along the weld path, the initial stage covering advancement of the laser beam along the beginning portion of the weld path and the final stage covering advancement of the laser beam along the ending portion of the weld path, wherein controlling the power level during at least one of the initial or final stages of advancing the laser beam along the weld path limits the line energy of the laser beam to being no greater than 10% above the line energy of the laser beam during an intermediate stage of advancing the laser beam along the weld path, the intermediate stage covering advancement of the laser beam along a middle portion of the weld path between the beginning and ending portions. 
 
     
     
       2. The method set forth in  claim 1 , wherein the first aluminum alloy workpiece has an outer surface and a first faying surface, and the second aluminum alloy workpiece has an outer surface and a second faying surface, the outer surface of the first aluminum alloy workpiece providing the accessible, top surface of the workpiece stack-up assembly and the outer surface of the second aluminum alloy workpiece providing a bottom surface of the workpiece stack-up assembly, and wherein the first faying surface of the first aluminum alloy workpiece and the second faying surface of the second aluminum alloy workpiece establish the faying interface. 
     
     
       3. The method set forth in  claim 1 , wherein the workpiece stack-up assembly further includes a third aluminum alloy workpiece that overlaps the second aluminum alloy workpiece, opposite the first aluminum alloy workpiece, to establish a second faying interface, the first aluminum alloy workpiece having an outer surface and a first faying surface, the third aluminum alloy workpiece having an outer surface and a fourth faying surface, and the second aluminum alloy workpiece having opposed second and third faying surfaces, the outer surface of the first aluminum alloy workpiece providing the accessible top surface of the workpiece stack-up assembly and the outer surface of the third aluminum alloy workpiece providing a bottom surface of the workpiece stack-up assembly, wherein the first faying surface of the first aluminum alloy workpiece and the second faying surface of the second aluminum alloy workpiece establish a first faying interface, wherein the third faying surface of the second aluminum alloy workpiece and the fourth faying surface of the third aluminum alloy workpiece establish the second faying interface, and wherein the keyhole and the molten aluminum alloy weld pool traverse both the first and second faying interfaces. 
     
     
       4. The method set forth in  claim 1 , wherein, during the intermediate stage of advancing the laser beam along the weld path, the power level of the laser beam is maintained at a target power level and the travel speed of the laser beam is maintained at a target travel speed. 
     
     
       5. The method set forth in  claim 4 , wherein the target power level is between 2 kW and 6 kW and the target travel speed is between 2 m/min and 5 m/min. 
     
     
       6. The method set forth in  claim 4 , wherein, during the initial stage of advancing the laser beam along the weld path, the laser beam is initially transmitted into the workpiece stack-up assembly and the power level of the laser beam is increased at a controlled rate up to the target power level while the travel speed of the laser beam along the weld path is accelerated up to the target travel speed. 
     
     
       7. The method set forth in  claim 6 , wherein the laser beam is initially transmitted into the workpiece stack-up assembly after a laser optic welding head that transmits the laser beam into the workpiece stack-up assembly has begun accelerating forward from a rest position. 
     
     
       8. The method set forth in  claim 4 , wherein, during the final stage of advancing the laser beam along the weld path, the power level of the laser beam is decreased at a controlled rate down from the target power level while the travel speed of the laser beam along the weld path is decelerated from the target travel speed, and transmission of the laser beam into the workpiece stack-up assembly is halted. 
     
     
       9. The method set forth in  claim 8 , wherein the transmission of the laser beam into the workpiece stack-up assembly is halted before a laser optic welding head that transmits the laser beam into the workpiece stack-up assembly has finished decelerating to a rest position. 
     
     
       10. The method set forth in  claim 1 , wherein the initial stage of advancing the laser beam along the weld path lasts from 0.3 seconds to 0.7 seconds, wherein the intermediate stage of advancing the laser beam along the weld path lasts from 1 second to 200 seconds, and wherein the final stage of advancing the laser beam along the weld path lasts from 0.3 seconds to 0.7 seconds. 
     
     
       11. The method set forth in  claim 1 , wherein the line energy of the laser beam during the intermediate stage of advancing the laser beam along the weld path is between 24,000 J/m and 180,000 J/m. 
     
     
       12. The method set forth in  claim 1 , wherein the line energy of the laser beam during at least one of the initial stage or the final stage of advancing the laser beam along the weld path is equal to or less than the line energy of the laser beam during the intermediate stage of advancing the laser beam along the weld path. 
     
     
       13. The method set forth in  claim 1 , wherein at least one of the first or second aluminum alloy workpieces comprises a non-heat-treatable aluminum alloy base layer that includes between 0.2 wt % and 6.2 wt % magnesium. 
     
     
       14. The method set forth in  claim 1 , wherein at least one of the first or second aluminum alloy workpieces comprises a heat-treatable aluminum alloy base layer that includes between 0.2 wt % and 3.0 wt % magnesium and 0.2 wt % and 1.8 wt % silicon. 
     
     
       15. The method set forth in  claim 1 , wherein at least one of the first or second aluminum alloy workpieces comprises a heat-treatable aluminum alloy base layer that includes between 0.8 wt % and 12 wt % zinc. 
     
     
       16. A method of laser welding a workpiece stack-up assembly that includes at least two aluminum alloy workpieces, the method comprising:
 (a) accelerating a laser optic welding head from a rest position; 
 (b) transmitting a laser beam from the laser optic welding head and into a workpiece stack-up assembly that includes at least two overlapping aluminum alloy workpieces, the laser beam having a power level and impinging an accessible top surface of the workpiece stack-up assembly within a welding region; 
 (c) increasing the power level of the laser beam at a controlled rate up to a target power level while the laser optic welding head is accelerating and the laser beam is being advanced relative to the accessible top surface of the workpiece stack-up assembly along a beginning portion of a weld path during which time a travel speed of the laser beam is accelerated up to a target travel speed; 
 (d) maintaining the power level of the laser beam and the travel speed of the laser beam at the target power level and the target travel speed, respectively, while the laser optic welding head is moving and the laser beam is being advanced relative to the accessible top surface of the workpiece stack-up assembly along a middle portion of the weld path; 
 (e) decelerating the laser optic welding head to a rest position; 
 (f) decreasing the power level of the laser beam at a controlled rate down from the target power level while the laser optic welding head is decelerating and the laser beam is being advanced relative to the accessible top surface of the workpiece stack-up assembly along an ending portion of the weld path during which time the travel speed of the laser beam is decelerated down from the target travel speed; and 
 (h) halting transmission of the laser beam from the laser optic welding head into the workpiece stack-up assembly; 
 wherein advancement of the laser beam along the weld path from the beginning portion to the ending portion forms a laser weld joint comprised of resolidified aluminum alloy workpiece material that penetrates through the workpiece stack-up assembly from the accessible top surface and at least across a faying interface established between the first and second aluminum alloy workpieces to fusion weld at least the first and second aluminum alloy workpieces together; 
 wherein the laser beam has a line energy, which is defined by the equation E L =P L /S L , wherein E L  is the line energy of the laser beam, P L  is the power level of the laser beam, and S L  is the travel speed of the laser beam; 
 wherein transmitting the laser beam from the laser optic welding head is delayed until after the laser optic welding head has begun accelerating from the rest position in step (a) to limit the line energy of the laser beam in the beginning portion of the weld path to no more than 10% greater than the line energy of the laser beam in the middle portion of the weld path, and/or halting transmission of the laser beam from the laser optic welding head occurs before the laser optic welding head has finished decelerating to the rest position in step (e) to limit the line energy of the laser beam in the ending portion of the weld path to no more than 10% greater than the line energy of the laser beam in the middle portion of the weld path. 
 
     
     
       17. The method set forth in  claim 16 , wherein the workpiece stack-up assembly further comprises a third aluminum alloy workpiece that overlaps and contacts the second aluminum alloy workpiece to establish a second faying interface within the workpiece stack-up assembly, and wherein a keyhole and a molten aluminum alloy weld pool traverse both the first and second faying interfaces such that the laser weld joint fusion welds the first, second, and third aluminum alloy workpieces together. 
     
     
       18. The method set forth in  claim 16 , wherein the power level of the laser beam is increased from below 0.05 kW up to the target power level at a controlled rate of 2.8 kW/s to 20 kW/s while, at the same time, the travel speed of the laser beam is accelerated up to the target travel speed at a rate of 170 m/min 2  to 1200 m/min 2 , and wherein the target power level is between 2 kW and 6 kW and the target travel speed of the laser beam is between 2 m/min and 5 m/min. 
     
     
       19. The method set forth in  claim 16 , wherein the target power level is between 2 kW and 6 kW and the target travel speed of the laser beam is between 2 m/min and 5 m/min, and wherein the power level of the laser beam is decreased from the target power level to below 0.05 kW at a controlled rate of 2.8 kW/s to 20 kW/s while, at the same time, the travel speed of the laser beam is decelerated from the target travel speed at a rate of 170 m/min 2  to 1200 m/min 2 . 
     
     
       20. A method of laser welding a workpiece stack-up assembly that includes at least two aluminum alloy workpieces, the method comprising:
 advancing a laser beam relative to an accessible top surface of a workpiece stack-up assembly that includes at least two overlapping aluminum alloy workpieces to form a laser weld joint, the laser beam being advanced along a weld path that includes a beginning portion, a middle portion following the beginning portion, and an ending portion following the middle portion, the laser beam having a power level and being advanced along the weld path at a travel speed, the laser beam having a line energy, which is defined by the equation E L =P L /S L , wherein E L  is the line energy of the laser beam, P L  is the power level of the laser beam, and S L  is the travel speed of the laser beam, and, wherein, during advancement of the laser beam along the middle portion of the weld path, the power level of the laser beam is maintained at a target power level and the travel speed of the laser beam is maintained at a target travel speed; 
 increasing the power level of the laser beam up to the target power level at a controlled rate during advancement of the laser beam along the beginning portion of the weld path while the travel speed of the laser beam along the weld path is accelerated up to the target travel speed, wherein increasing the power level of the laser beam up to the target power level at a controlled rate keeps the line energy of the laser beam along the beginning portion of the weld path equal to or less than the line energy along the middle portion of the weld path; and 
 decreasing the power level of the laser beam down from the target power level at a controlled rate during advancement of the laser beam along the ending portion of the weld path while the travel speed of the laser beam along the weld path is decelerated from the target travel speed, wherein decreasing the power level of the laser beam down from the target power level at a controlled rate keeps the line energy of the laser beam along the ending portion of the weld path equal to or less than the line energy along the middle portion of the weld path; 
 wherein the laser weld joint is comprised of resolidified aluminum alloy workpiece material that penetrates through the workpiece stack-up assembly to fusion weld the at least two overlapping aluminum alloy workpieces together, the weld joint being narrower at the accessible top surface of the workpiece stack-up assembly within the beginning portion and the ending portion of the weld path compared to the middle portion of the weld path due to the line energy of the laser beam along the beginning portion of the weld path and the line energy of the laser beam along the ending portion of the weld path both being equal to or less than the line energy of the laser beam along the middle portion of the weld path.